The invention relates to an EUV mirror, to an optical system comprising an EUV mirror, and to a method for operating an optical system. One preferred field of application is EUV microlithography. Other fields of application are in EUV microscopy and EUV mask metrology.
Nowadays predominantly microlithographic projection exposure methods are used for producing semiconductor components and other finely structured components. In this case, use is made of masks (reticles) or other patterning devices which carry or form the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object surface of the projection lens and illuminated with an illumination radiation provided by the illumination system. The radiation altered by the pattern passes as projection radiation through the projection lens, which images the pattern onto the substrate to be exposed, which is coated with the radiation-sensitive layer.
The pattern is illuminated with the aid of an illumination system, which, from the radiation from a primary radiation source, forms an illumination radiation which is directed onto the pattern and which is characterized by specific illumination parameters and impinges on the pattern within an illumination field of defined form and size. Within the illumination field, a predetermined local intensity distribution should be present, which is normally intended to be as uniform as possible.
In general, depending on the type of structures to be imaged, different illumination modes (so-called illumination settings) are used, which can be characterized by different local intensity distributions of the illumination radiation in a pupil surface of the illumination system. It is thereby possible to predetermine in the illumination field a specific illumination angle distribution or a specific distribution of the impinging intensity in the angle space.
In order to be able to produce ever finer structures, various approaches are pursued. By way of example, the resolution capability of a projection lens can be increased by enlarging the image-side numerical aperture (NA) of the projection lens. Another approach consists in employing shorter wavelengths of the electromagnetic radiation.
If it is attempted to improve the resolution by increasing the numerical aperture, then problems can arise by virtue of the fact that as the numerical aperture increases, the depth of focus (DOF) that can be achieved decreases. This is disadvantageous because a depth of focus of the order of magnitude of at least 0.1 nm is desirable for example for reasons of the achievable flatness of the substrates to be structured and mechanical tolerances.
For this reason, inter alia, optical systems have been developed which operate with moderate numerical apertures and achieve the increase in the resolution capability substantially by means of the short wavelength of the used electromagnetic radiation from the extreme ultraviolet range (EUV), in particular having operating wavelengths in the range of between 5 nm and 30 nm. In the case of EUV lithography having operating wavelengths of around 13.5 nm, for example given image-side numerical apertures of NA=0.3 it is theoretically possible to achieve a resolution of the order of magnitude of 0.03 μm in conjunction with typical depths of focus of the order of magnitude of approximately 0.15 μm.
Radiation from the extreme ultraviolet range cannot be focused or guided with the aid of refractive optical elements, since the short wavelengths are absorbed by the known optical materials that are transparent at higher wavelengths. Therefore, mirror systems are used for EUV lithography. A mirror (EUV mirror) having a reflective effect with respect to radiation from the EUV range typically has a substrate, on which is applied a multilayer arrangement having a reflective effect with respect to radiation from the extreme ultraviolet range (EUV) and having a large number of layer pairs comprising alternately low refractive index and high refractive index layer material. Layer pairs for EUV mirrors are often constructed with the layer material combinations molybdenum/silicon (Mo/Si) or ruthenium/silicon (Ru/Si).
In order to ensure a best possible uniformity of the lithographic imaging, it is generally endeavoured to produce an intensity distribution that is as uniform as possible in the illumination field illuminated by the illumination system. Furthermore, it is normally endeavoured to approximate the local intensity distribution of the illumination rays that is desired for a specific exposure in the pupil surface of the illumination system as exactly as possible to the desired spatial intensity distribution or to minimize deviations from a desired spatial intensity distribution. These requirements not only have to be met by the lithographic optical system at the time of its delivery, but have to be maintained over the entire lifetime of the optical system without significant change. While in the former case possible deviations are substantially based on design residues and manufacturing faults, changes over the lifetime are often substantially caused by ageing phenomena.
In optical systems for lithography using ultraviolet light from the deep or very deep ultraviolet range (DUV or VUV), non-uniformities that possibly arise can generally be compensated for by driveable mechanical compensators (cf. e.g. US 2008/113281 A1 or U.S. Pat. No. 7,545,585 B2).
In optical systems for EUV microlithography, such compensators are significantly more difficult to realize, inter alia for geometrical reasons. By way of example, a freely accessible intermediate field plane which is optically conjugate with respect to the object plane of the projection lens and in which the field homogeneity can be corrected in a simple manner often does not exist. WO 2010/049020 A1 discloses possibilities for correcting the illumination intensity distribution and the illumination angle distribution in the illumination field of an EUV illumination system. Other correction devices are disclosed in US 2003/0063266 A1, EP 1 349 009 A2, US 2008/0165925 A1 or WO 2009/135576 A1.